3
Addressing
58
ARP
Address
Resolution
Protocol
(ARP)
allows
the
mapping
of
a
network
layer
protocol
(OSI
Layer
3)
address
to
a
data
link
layer
hardware
address
(OSI
Layer
2).
In
data
networks
it
is
used
to
resolve
an
IPv4
address
into
its
corresponding
Ethernet
address.
ARP
operates
at
the
OSI
Layer
2,
data
link
layer,
and
is
encapsulated
by
Ethernet
headers
for
transmission.
For
an
overview
of
the
different
OSI
layers,
see
OSI
Model
on
page
197
.
IP addressing over Ethernet
A
host
in
an
Ethernet
network
can
communicate
with
another
host
only
if
it
knows
the
Ethernet
address
(MAC
address)
of
that
host.
Higher
level
protocols
such
as
IP
make
use
of
IPv4
addresses
that
are
fundamentally
different
from
a
lower
level
hardware
addressing
scheme
like
the
MAC
address.
ARP
is
used
to
retrieve
the
Ethernet
MAC
address
of
a
host
by
using
its
IP
address.
ARP resolution
When
a
host
needs
to
resolve
an
IPv4
address
to
the
corresponding
Ethernet
address,
it
broadcasts
an
ARP
request
packet.
The
ARP
request
packet
contains
the
source
MAC
address,
the
source
IPv4
address,
and
the
destination
IPv4
address.
Each
host
in
the
local
network
receives
this
packet.
The
host
with
the
specified
destination
address
sends
an
ARP
reply
packet
to
the
originating
host
with
its
MAC
address.
SEG ARP cache
The
ARP
cache
in
network
equipment,
such
as
switches
and
security
gateways,
is
an
important
component
in
the
implementation
of
ARP.
It
consists
of
a
dynamic
table
that
stores
the
mappings
between
IPv4
addresses
and
Ethernet
MAC
addresses.
The
SEG
uses
an
ARP
cache
in
exactly
the
same
way
as
other
network
equipment.
Initially,
the
cache
is
empty
at
SEG
startup
and
becomes
populated
with
entries
as
traffic
flows.
The
typical
contents
of
a
minimal
ARP
cache
table
might
look
similar
to
the
following:
The
explanation
for
the
table
contents
is
as
follows:
•
The
first
entry
in
this
ARP
cache
is
a
dynamic
ARP
entry
which
tells
us
that
IPv4
address
192.168.0.10
is
mapped
to
an
Ethernet
address
of
08:00:10:0f:bc:a5
.
•
The
second
entry
in
the
table
dynamically
maps
the
IPv4
address
193.13.66.77
to
Ethernet
address
0a:46:42:4f:ac:65
.
•
The
third
entry
is
a
static
ARP
entry
binding
the
IPv4
address
10.5.16.3
to
Ethernet
address
4a:32:12:6c:89:a4
.
Type
IPv4 address
Ethernet address
Expires
Dynamic 192.168.0.10
08:00:10:0f:bc:a5
45
Dynamic 193.13.66.77
0a:46:42:4f:ac:65
136
Static 10.5.16.3
4a:32:12:6c:89:a4
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